Not Applicable
Not Applicable
1. Technical Field
This invention relates to electronic devices of the type having electrical components soldered together. The invention further relates to such devices in which one or more of the components are protected with an environmentally shielding encapsulant before soldering operations are performed. The invention thus relates to electronic devices that incorporate one or more integrated circuit chips and carrier assemblies by solder connection of their electrical leads to contacts connected or connectable to other circuitry in the device. In particular the invention relates to manufacture of such devices using heated, flowing solder to bond the electrical leads to the contacts while protecting the encapsulants typically on the assemblies from degradation from exposure to the heat of the solder.
2. Description of the Related Art
Electronic devices herein refers to the combination of integrated circuit chips (IC""s) and supports therefor, including those supports commonly referred to as circuit boards. Generally electronic devices comprise a structure incorporating an IC bonded to a carrier as the support. The IC has incoming and outgoing (I/O) electrically conductive leads to electrically connect itself to the carrier as an I/C/carrier assembly. The IC can be any of a number of standard semiconductor devices, including without limitation flip-chips, ball-grid arrays, chip-on-board assemblies or tape-automated-bonded ICs. The IC can be bonded to a carrier (or lead frame) having mating incoming and outgoing electrically conductive leads. The carrier is typically composed of FR4 epoxy boards, bis-maleimide circuit boards, copper lead frames, Kovar lead frames, flexible circuitry or various ceramic substrates, such as aluminum nitride ceramics. The carrier will contain electrically conductive laminar leads typically having fine pitch. The material bonding the IC to the carrier can be organic such as any suitable thermosetting or thermoplastic polymer, some containing thermally conductive or electrically conductive fillers, or the bonding material can be inorganic and composed of a solder, solder paste, glass, some with thermally or electrically conductive ingredients contained within a glass. After assembly of the IC and carrier, the IC is electrically connected to the carrier using wire-bonding techniques utilizing gold or aluminum wire, or utilizing bumped chips soldered or bonded to a grid array. Once mechanically and electrically assembled, the IC and carrier assembly is protected from the environment by encapsulation with a suitable encapsulant for eventual use in an electronic device. In creating the electronic device, the encapsulated assembly leads are juxtaposed with device contacts connected to the device circuitry and soldered therewith to form conductive junctions by dipping or otherwise flowing solder over the juxtaposed contacts and leads. Failure of the encapsulant to survive the 20 to 120 seconds of heating in direct contact with molten solder, or indirect contact therewith by exposure to the heat of the molten solder, as manifested by degradation of the encapsulant through debonding, softening, increased tackiness or apparent gumminess exposes the electronic device to environmental problems and possible fatal contamination.
It is an object therefore to provide an improved electronic device able to survive solder heat exposure during soldering cycles. It is another object to provide a method of manufacturing electronic devices incorporating encapsulated components in which the components are protected from environmental damage by an encapsulant resistant to soldering cycles. It is a further object to provide such devices and methods in which their component encapsulants have predominately thermodynamic polymer portions over thermostatic polymer portions so as to endure soldering temperature cycles without degradation. It is a still further object to provide to the IC assembly with a carrier a mass that contains within it a thermal well sufficient to incorporate the heat flux generated during solder heat exposure. It is yet another object to provide structure in which the thermal wells function by incorporating the heat flux by randomizing or ordering polymer chains rather than in the form of internal energy, in which there is sufficient mass capacity and in which the thermal wells function to capture the intensity of the heat flux for a period sufficient to allow the encapsulant to withstand bond-breaking and disintegration while, at the same time, reducing stresses sufficient to maintain the integrity of the entire encapsulant assembly during the solder dipping process and during cool-down to room temperature. A further object is to attach the entire encapsulated IC/carrier assembly to main circuitry to be soldered, e.g. by being passed over wave solder to create the solder contacts with the main circuitry while maintaining a heat-stable, encapsulated and hermetically-sealed integrated circuit structure, so as to form the invention electronic device.
These and other objects of the invention to become apparent hereinafter are realized in the method for the manufacture of an electronic device including putting through a hot solder cycle for electrical connection to each other a carrier contact and a lead of an integrated circuit chip having a predetermined physical geometry which is substantially constant in the presence of the hot solder, and during the course of such hot solder cycle maintaining about the chip in heating contact with the solder a protective composition congruent with and adhered to the chip, the protective composition comprising a resin subject to expansion with the heat of the solder, the resin having crystalline thermostatic polymer chain portions and thermodynamic amorphous polymer chain portions, the proportion of the thermodynamic polymer chain portions to the total of polymer chain portions being sufficient to absorb the expansion of the resin in the presence of solder heat without concurrent expansion of the thermostatic polymer chain portion so as to maintain the congruity with and adherence to the chip of the protective composition through the hot solder cycle.
The invention further provides the product of the foregoing method and more generally an electronic device comprising a carrier having a contact and an integrated circuit chip of a given physical geometry, the chip having a lead electrically connected in hot-soldered relation to the contact, the chip having a protective composition congruent therewith and adhering thereto, the composition comprising a resin having thermostatic crystalline polymer chain portions and thermodynamic amorphous polymer chain portions, the thermodynamic amorphous polymer chain portions being present in such relative proportions that the thermostatic polymer chain portions remain congruent and adherent to the chip in their locus of original application after solder heat contact.
The invention electronic device and method typically employs as the resin the polymerization reaction product of a first reactive component formed of an aliphatic diisocyanate prepolymer and an oligomer reacted with an aliphatic diisocyanate and a second reactive component formed of an aliphatic or aromatic diamine and an oligomer, the oligomer comprising a generally straight-chain polymeric moiety having a molecular weight between about 100 and 20,000 daltons and substituted on about every second to fifteenth in-chain carbon atom and effective to produce three-dimensional twisting and winding atactically, syndiotactically or isotactactically in the resin; first reactive component prepolymer aliphatic diisocyanate comprises methylene dicyclohexane diisocyanate, isophorone diisocyanate, and hexamethylene diisocyanate, the prepolymer being comprised of from 3 to 50% by weight of the diisocyanate, the prepolymer being reacted with methylene dicyclohexane diisocyanate, isophorone diisocyanate, or hexamethylene diisocyanate present in an amount from 5 to 50% by weight of the first reactive component;
The oligomer comprises active hydrogen functional polymerized linear, cyclic or branched alkanes and alkenes, and alkanes or alkenes polymerized with alkenes or alkanes respectively or alkynes, e.g. homo-and co-polymers of ethylene, propylene, butylene, vinyl, allyl, chlorinated vinyl, or diene monomers, oligomers and polymers, such as polyvinyl chloride, ethylene polymers, propylene polymers, dienes, ethylene-propylene polymers, polyisoprenes, natural rubbers, polybutylene polymers, styrenebutadiene polymers, or halogenated polymers, and preferably polymers of 1,3-butadiene as the oligomer, the polymers having a molecular weight of less than about 4000 daltons.
The second reactive component employs the oligomer comprising a generally straight-chain polymeric moiety having a molecular weight between about 100 and 20,000 daltons and substituted on about every second to fifteenth in-chain carbon atom and effective to produce three-dimensional twisting and winding atactically, syndiotactically or isotactactically in the resin; the second reactive component also employs as the amine one comprising a primary or secondary aromatic or aliphatic diamine, preferably selecting as the amine an aromatic diamine, selecting as the diamine a primary or secondary aromatic or aliphatic diamine comprising a diamino alkane, or an alkyl, alkoyl, aryl, aroyl, or alicyclic-substituted diamino alkane, or specifically selecting the amine from ethylene diamine, piperazine, n-aminoethyl piperazine, diethylene triamine, triethylene tetramine, piperazine cyclics, 1,3-bis(aminoethyl) cyclohexane, 1,4 diaminocycylohexane, m-xylene diamine, homologues thereof, and amino-capped low molecular weight polyols, preferably the diamine having a molecular weight of less than about 2000 daltons, and being present in an amount of less than about 10% by weight of the second reactive component.
The invention method and product further contemplates using an aromatic diamine, such as a primary or secondary aromatic amine having di- or multifunctionality and a molecular weight of less than about 2000 daltons, e.g. 3,5-diethyl-2,4-toluene diamine, di-(3,5-methyl thio)-2,4-toluene diamine, methylene-bis-orthochloro aniline, methylenedianiline, methylene-bis-methyl anthranilate, m-phenyl diamine, trimethylene glycol-di-p-amino benzoic ester, or amine capped polyols.